A 5,000-mile-long ‘river in the sky’ is slamming Pacific Northwest with heavy rain and snow

A fascinating weather pattern is in place in the Northern Hemisphere this week. It’s a river in the sky that spans the full width of the Pacific Ocean from Asia to North America. The phenomenon is remarkably obvious — a plume of clouds with a few areas of low pressure embedded, transported at shocking speeds across the planet’s largest ocean via the jet stream.

Plainly spoken, it’s a train of rain on its way to Washington State.

Over the coming days, this so-called ‘atmospheric river’ will unload tremendous amounts of rain and snow over the Pacific Northwest. The series of storms will spray out the equivalent of 10 to 15 inches of rain at high elevations and up to several inches near the coast, including over Seattle and Portland. On the high peaks of the Cascades, heavy snows — measured in feet — are predicted.

Wednesday will likely be Seattle’s wettest 24 hours since February, the National Weather Service said. More than an inch of rain is in the forecast through early Thursday morning. The record for Oct. 18, at Seattle-Tacoma International is 1.19 inches in 1958.

Widespread rainfall totals of 2 to 5 inches are in the forecast through Sunday.

Rainfall forecast for the Seattle region though Sunday night. (weatherbell.com)

Snow levels in the Cascades are initially expected to be very high, above 10,000 feet, but lower as the week wears on. On Mount Baker (elevation around 10,000 feet), the Weather Service is predicting a remarkable 87 to 111 inches (7 to 9 feet) of snow through Thursday — with more after that.

SnowBrains, a website for ski enthusiasts, reports that 60 to 80 inches of snow could fall on the high elevations at the Whistler ski resort in British Columbia.

Atmospheric rivers transport up to half of the West Coast’s precipitation each year during the rainy season, which stretches from October to April.

The atmospheric river set to arrive in Washington state Wednesday is this rainy season’s first in the Pacific Northwest. A second — part of the same cloud complex sprawled across the Pacific — is predicted to follow Saturday, probably targeting Oregon.

Imagery of the cloud swath stretched across the entire Pacific, along which these rivers are flowing, has stunned meteorologists.

F. Martin Ralph, an expert on atmospheric rivers at Scripps Institution of Oceanography, said the length of the cloud complex is unusual but not without precedent. He pointed to a similar cloud complex and atmospheric river set up over the Pacific in October 2009, aimed at central California, which dumped up to 15 inches of rain.

“This event is looking a like a pair of significant atmospheric rivers over 4 to 5 days,” Ralph said in an interview. “The details of where they make landfall and how long they last will determine where the heaviest precipitation and impacts will occur.”

GFS model showing where plumes of water vapor — representing atmospheric rivers — are concentrated over the Pacific on Wednesday. (Scripps)

The rivers are moving along a zone of rip-roaring upper-level winds known as the jet stream. “The jet stream (ribbon of high wind above 30,000 feet) will be a major player in the week ahead — exceeding 200 mph (180 knots) at times,” the Weather Service in Seattle tweeted.

In addition to the heavy rainfall, the Weather Service said the incoming weather system Wednesday will bring strong winds, up to 60 mph at the coast, and 40 to 45 mph in Seattle. “Local power outages can be expected as trees get pruned of their dead branches,” it said.

While clouds covered a long ribbon of the Pacific Ocean Tuesday, hardly a cloud could be found over the contiguous U.S. — for the time being …